Many printing machines are designed to create an image by placing a series of picture elements (pels) on the image receiving material. For example, in electrophotographic printing machines, an image may be created by a light source which is caused to scan across photosensitive material in a succession of scan lines. The light beam places a series of overlapping pels on the charged surface of photosensitive material. Each pel is placed in a pel area and the light beam is modulated so that some pel areas are exposed to light and some are not. Wherever a pel containing light strikes the photosensitive material, it is discharged. In that manner, the photosensitive material is caused to bear a charge pattern of pels which images the subject that is being reproduced. The printed copy is obtained by developing the charge pattern and transferring the developed image to print material, usually paper.
The resolution of images produced by a laser electrophotographic machine is generally stated in the number of pels produced per inch. For example, a 300-pel per inch electrophotographic printer has higher resolution than a 240-pel per inch (about 100 pels per cm) printer. While the visual characteristics of print are generally better when a higher number of pels per inch are used, in one area the visual characteristics are made worse. That area is the printing of narrow fine lines, for example, lines of a single pel width. The reason is that as the number of pels per inch increase, the width of a single pel decreases. This decrease in pel width is made even more severe by the overlapping pel structure used in electrophotographic printing machines. The invention of U.S. Pat. No. 4,544,264, filed concurrently herewith, is apparatus and technique to cause the enhancement of the printing of fine lines such as lines of a single pel width and to provide for such enhancement in two dimensions. That is, the printing of fine lines in a direction perpendicular to the scan direction is enhanced as well as the printing of fine lines which occur in a direction parallel to the scan direction.
At a resolution of 240 pels per inch, lines printed parallel or perpendicular to scan print with very little visible distortion. However, diagonal lines print with a staircase distortion which is quite visible to the human eye. U.S. Pat. No. 4,460,909 describes apparatus and tecnnique for smoothing the visible digitization present in diagonal lines.